The electrophoretic display offers an electronic alternative to conventional printed-paper media for many applications. The electrophoresis phenomenon is based on charged particles suspended in a liquid fluid, for example charged pigment particles in an organic solvent. Unlike sheet materials containing magnetic memory areas that can be written electronically, an electrophoretic display advantageously provides a visible record for the viewer.
Electrophoretic media systems exist that maintain electronically changeable data without power, such as devices available from E-ink Corporation, Cambridge, Mass., or GYRICON systems from Xerox Corporation, Stamford, Conn.
As initially proposed in the late 1960's, the electrophoretic display typically comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes, for placement nearer to the viewer, is usually transparent. In one prior-art embodiment, a fluid suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side, such that either the color of the pigment or the color of the solvent is predominant, depending on the polarity of the voltage difference.
Since the inception of this technology, there has been considerable research directed to its implementation and optimization. For example, in order to prevent undesired movement of the particles, such as sedimentation or lateral migration, partitions between the two electrodes were proposed for dividing the space into smaller cells. However, in the case of partition-type electrophoretic displays, difficulties were encountered in the formation of the partitions, in the process of enclosing the fluid suspension, and in the case of colored displays, in segregating different colored fluid suspensions from each other in partition-type electrophoretic displays.
Partitioning electrophoretic displays into smaller cells has been accomplished by a photolithographic process. This is a batchwise process requiring solvent development step. However, it is desirable to be able to manufacture electrophoretic displays without requiring a solvent development step, and there is a need for an improved processing method that provides high throughput, especially a continuous process.
Roll-to-roll microembossing processing has been proposed as an alternate fabrication method for cell formation in an electrophoretic display. For example, U.S. Pat. No. 6,831,770 B2 entitled “Electrophoretic Display and Novel Process for its Manufacture” to Liang et al. discloses an electrophoretic display comprising microcells, termed microcups, that are filled with charged particles dispersed in a solvent, wherein each cell is individually sealed with its own polymeric sealing layer or cap. The polymeric seal in each microcup is obtained by adding a separate sealing composition to each individual microcup along with the charged pigment dispersion comprising pigment particles. The sealing composition can be thoroughly blended with the electrophoretic fluid containing charged pigment particles dispersed in a colored dielectric solvent. For example, an in-line mixer or other blending apparatus can be used to mix the two materials. Then, the mixture can be immediately coated onto a sheet having microcups, using a precision coating mechanism such as Myrad bar, gravure, doctor blade, slot coating or slit coating. Notably, the sealing composition is immiscible or otherwise incompatible with the solvent and has a lower specific gravity than the solvent and pigment particles. The sealing composition in each microcup forms a supernatant layer, on top of the charged pigment dispersion, that is hardened using radiation, heat, moisture, or some other means in order to form the seal. In this way, each microcup becomes a separately sealed container with an electrophoretic fluid mixture. The process described by Liang et al. can be, for example, a continuous roll-to-roll process, as shown in FIG. 6 of the afore-mentioned U.S. Pat. No. 6,831,770 B2.
Thus, the method of the Liang et al. involves individually sealing the microcup cells, that is, the sealing layer for each microcup can be formed as an individual seal that is discontinuous with the sealing layer for each of the surrounding microcups in an array of microcups, and a common sealing layer does not seal a plurality of microcups. Once the microcups are individually sealed, only then is a lamination sheet applied over the microcups, wherein the lamination sheet is a second conductor film pre-coated with an adhesive layer.
In an alternate embodiment disclosed by Liang et al. in U.S. Pat. No. 6,831,770 B2, a sealing layer can be formed by overcoating the microcups, once filled with electrophoretic fluid, with a thin layer of a sealing composition.
Liang et al. state that the sealing layer may extend over the top surface of the cell side walls (FIG. 8), thereby forming a stopper-shaped sealing layer having a thickness ranging from about 0.1 μ to about 50 μm, in which the cell is only partially filled with the electrophoretic fluid. The thickness of the sealing layer below the top surface of the partition walls and above the interface is at least 0.01 μm above the interface. It is preferred that the sealing layer forms a contiguous film above the cell walls and the electrophoretic fluid. Liang et al. state that the cell is sandwiched between two conductive layers and that there may be an additional adhesive layer between the top of the sealing layer and the top conductive layer.
Thus, in the top-sealing process of Liang et al., the display fluid is enclosed and top-sealed before a second substrate or electrode layer is disposed or laminated onto the display cells. Also, the seal must be formed with a lower density material. Among the problems with this type of sealing arrangement are difficulties in preventing or minimizing the degree of intermixing between the sealing composition and the pigment dispersion. Also, it is difficult to adjust the thickness of the seal, which needs to be much less than that of the electrophoretic fluid in order to provide the necessary optical density. Consequently, the specific gravity and viscosity of the materials must be carefully controlled, which limits the materials that can be used. Volatile solvents and fluorinated compounds may be used to adjust properties such as the viscosity and the thickness of the coatings. However, this adds complexity and further steps to the fabrication process.
U.S. Pat. No. 6,940,634 entitled “Electrophoretic Display Device” to Ukigaya describes the prior-art manufacture of an electrophoretic display device by adhering a substrate and a microcell sheet together through an adhesive layer, wherein the top surfaces of the microcell walls are adhered to the substrate (see column 1). Ukigaya states that production steps and apparatus for forming the adhesive layer are complicated and can cause yield reductions or increased production costs. As a solution, Ukigaya proposes using an electrode having adhesive properties. An adhesive electroconductive resin is applied between the partition wall of a microcell and a protective substrate, in order both to eliminate the prior-art adhesive layer and to provide electrical connections to each cell. This type of approach may prove useful for some types of electrophoretic cell design, but would offer no advantage for an in-plane electrode design, in which all electrodes lie in the same plane within the electrophoretic cell. Also, the requirement for electrical conductivity of the adhesive significantly narrows the choices of adhesive material available, and steps taken to provide or enhance resin conductivity using metallic powders or other particulates could cause disadvantageous optical effects in the display.
In yet another prior-art approach, U.S. Patent Application Publication No. 2005/0122565 A1 entitled “Electrophoretic Displays and Materials for Use Therein” by Doshi et al. discloses an adhesive layer disposed between the first and second substrate of an electro-optical display. Doshi et al. disclose, in forming the final display, laminating a first substrate having s a layer of encapsulated electrophoretic medium (capsules in a binder) to a second substrate, a backplane, using a lamination adhesive (paragraph 0018). Doshi et al. state that such a process allows for mass production of displays by roll lamination. However, in this process, the electrophoretic medium is first dried to form a coherent layer of the electrophoretic medium firmly adhered to the first substrate before the lamination step. Doshi et al. also state that similar manufacturing techniques can be used with other types of electro-optic displays. For example, a microcell electrophoretic medium may be laminated to a backplane. However, Doshi et al. do not describe how to laminate a backplane to an unsealed microcell sheet containing a flowable liquid fluid as compared to a dried, coherent layer. Doshi et al. also disclose the use of vacuum lamination; however, this would be inappropriate with liquid or other materials that are not already bonded to an underlying substrate in some way.
U.S. Patent Publication No. 2005/0133154 entitled “Method of Sealing an Array of Cell Microstructures Using Microencapsulated Adhesive” to Daniel et al. states that one known method of sealing microcells involves providing a wall microstructure on a first flexible substrate, coating a second flexible substrate with a substantially continuous layer of adhesive or sealant, and positioning the second flexible substrate on the end portion of the wall microstructure (apparently the top surface of the side walls) to effectively seal the microcells. Daniel et al., however, point out disadvantages of applying a continuous layer of adhesive onto a substrate for bonding to cell walls for sealing the electrophoretic microcells. Notably, excess liquid adhesive that is not used in forming the bond with the wall microstructure of the microcells tends to migrate into or otherwise intermix with the contents of the cells. This unwanted mixing can undesirably affect properties of the electrophoretic substance contained within the cells. To overcome this problem, Daniel et al. disclose a method of sealing an array of cell microstructures using a microencapsulated adhesive. In one particular embodiment (as shown in FIG. 3 of the Daniel et al. disclosure), a second substrate having a plurality of adhesive microcapsules supported on a first side of the second substrate is displaced against portions of the cell microstructure on a first substrate. As the second substrate approaches the wall microstructure, a portion of the microcapsules are compressively captured between opposing contact points and rupture, thereby locally dispensing the adhesive contained therein. Consequently, each individual cell is substantially sealed by a locally released adhesive substance and any remaining adhesive microcapsules simply remain, sealed and trapped within the fully enclosed and sealed cells. With this type of approach, care must be taken to distribute the microcapsules suitably for obtaining sufficient levels of localized adhesion. Of course, for using such an approach, the microcapsules themselves must be fabricated from materials and in shapes that are compatible with the light-handling requirements of the electrophoretic device. Cell walls themselves must allow sufficient compression force to break open the compressively captured portion of the microcapsules for sealing. Thus, while this type of approach may prove useful for some cell sealing applications, there are drawbacks and limitations to such a solution for the broad range of electrophoretic and other electro-optical modulating image-forming applications.
In view of the above, conventional approaches for sealing an array of microcells in an electro-optical modulating display have not provided solutions that are sufficiently adaptable and robust for large-scale production. Among the problems that have not been adequately addressed are difficulties due to the composition of the electrophoretic fluid itself. Many of the liquids and solvents used can even prove inimical to conventional surface adhesion materials and techniques or, at best, allow only marginal performance. Self-capping approaches such as those taught by Liang et al. and elsewhere place constraints on both the electrophoretic composition and on the sealing materials themselves. The adhesive electrode taught in the Ukigaya disclosure allows only a narrow range of materials and is not appropriate for designs using an in-plane electrode layout. The method of Doshi et al., involving lamination, may be suitable for electrophoretic composite materials that are not fluid in nature, but do not satisfy the more demanding requirements posed by microcells containing a liquid electrophoretic medium. Finally, adhesive capsules as proposed by Daniel et al. allows only a narrow range of adhesives, may pose limitations due to cost and suitability for mass manufacture, and may result in a residue of material in the microcells that can adversely effect optical performance.
Still another problem with sealing an array of microcells, particularly when simultaneously laminating and sealing an array of liquid-containing microcells is to prevent or limit the entrapment of any air while sealing the microcells, since air bubbles formed in sealed microcells will result in undesirable variations in image density among the microcells.